This invention relates to a gelable composition which has different crosslinkers therein which crosslink a water-dispersible polymer in the composition at corresponding different temperature ranges. In another aspect, the invention relates to use of this composition in steam treatment of wells for enhanced oil recovery.
Steam treatment of wells has been practiced for some time in order to heat especially heavy crudes in a subterranean formation so as to reduce the crude viscosity and enhance oil recovery. Many problems have been encountered in such steam treatments, however. One problem results from the typically varying permeability of different strata in the formation. The injected steam will tend to flow into the more permeable portions (hereafter referred to as "thief zones") of the formation, thereby leaving other possibly oil bearing zones untouched.
Various methods of plugging or sealing off thief zones for diverting steam into the less permeable zones have been used. One method involves surfactants which are added to injected steam to form steam foams which plug or increase the flow resistance of thief zones. The desired diversion of steam away from thief zones, however, is achieved for only a short time after the steam injection. As the steam cools and condenses, the foam tends to collapse and the desired effect is lost. Other chemicals have been proposed for blocking their zones, but few are economically attractive or provide good penetration into a formation, and if a permanent blockage of thief zones is desired, few chemicals can withstand the effects of high steam temperatures over prolonged periods. For example, lignosulfonate forms a gel that can block thief zones, but it loses its effectiveness with prolonged exposure to high temperatures.
In summary, the existing technology of steam diversion from thief zones is inadequate insofar as prior thief zone sealants are effective in blocking thief zones for an undesirably short period of time after steam injection, or fail to penetrate deep enough into the formation.